1. Field of the Invention
The present invention relates to an optical amplification apparatus for amplifying signal light using Raman amplification, and in particular to an optical amplification apparatus having a function for detecting an input interruption of signal light.
2. Description of the Related Art
Recently, the development of techniques has been progressed for achieving for example an expansion of optical amplification bands, or a reduction in repeater loss in various types of optical transmission systems, through the construction of optical amplification apparatus making use of Raman amplification. For example, an optical amplification apparatus is proposed with a construction as shown in FIG. 10, where a Raman amplifier is disposed prior to for example an erbium doped optical fiber amplifier (EDFA), and Raman amplified signal light is input to the EDFA. Furthermore, in the future, it may be considered that a Raman amplifier alone constructs the optical amplification apparatus.
However, with a general optical transmission system which repeatedly transmits signal light using an optical amplification apparatus, for example in the case where the signal light is cut off due to the occurrence of an open circuit of the transmission path or a disconnection of a connector, it is necessary to instantly detect the input interruption of the signal light in the optical amplification apparatus. The reason why such input interruption detection is necessary is to avoid for example; a problem where, since an AGC for controlling an amplification gain of signal light to be constant, or an ALC for controlling the level of output light to be constant, are generally applied to an optical amplification apparatus, in the case of an input interruption in the signal light, an amplification operation is controlled so as to obtain a predetermined output light by only a noise component which is generated in the optical amplification apparatus, or a problem where when the input interruption of the signal light is recovered in such a condition, in the case where an EDFA is used as the optical amplification apparatus, a surge is generated bringing damage to the apparatus.
With the optical amplification apparatus using an EDFA, a so-called shutdown control has been performed which detects an input interruption of the signal light and shuts off supply of excitation light to the erbium doped fiber (EDF). More specifically, for example as shown in FIG. 11, when wavelength division multiplexed (WDM) signal light sent from a prior stage EDFA (not shown in the figure) via a transmission path is to be collectively amplified by an EDFA, a part of the WDM signal light input to the EDFA is branched by an optical coupler, and the power of the branched light is monitored by an light power monitor section. The light power monitored by the light power monitor section becomes, for example as shown in FIG. 12A, the light power corresponding to the sum of a signal light component contained in the WDM signal light and amplified spontaneous emission light (ASE light) which is generated and accumulated in the prior stage EDFA and so forth.
With such a construction, if an input interruption of the WDM signal light occurs due for example to an open circuit of the transmission path connected to the prior stage EDFA, a disconnection of a connector, or the like, then as shown in FIG. 12B, the light power monitored by the aforementioned light power monitor, becomes approximately zero. Consequently, with the shutdown control in the conventional EDFA, in the case where the light power monitored by the light power monitor section falls to a predetermined threshold value or below, the EDFA control section judges an input interruption of the WDM signal light to perform a control to shut down the supply of the excitation light to the EDFA.
In the case where the above described conventional EDFA shutdown control is applied to an optical amplification apparatus as shown in the aforementioned FIG. 10 where a Raman amplifier and an EDFA are combined, then caused by the generation of noise light due to Raman amplification, there is a problem that it is difficult to accurately judge an input interruption of the signal light. This noise light due to Raman amplification, is noise light which is also generated in the case where, in a situation where the signal light is not input, Raman excitation light only is emitted into an amplifying medium, and in general is referred to as Raman scattering light due to pumping light. Here, in contrast to the amplified spontaneous emission (ASE) light generated in the EDFA, the abovementioned noise light generated in the Raman amplifier is referred to as amplified spontaneous Raman scattering (ASS) light.
In an optical amplification apparatus where a Raman amplifier and an EDFA are combined, the power of the input light to the EDFA, to be monitored by the light power monitor is, for example as shown in FIG. 13A, specifically the light power corresponding to the sum of the signal light component, the ASE light component which is generated and accumulated in the prior stage EDFA and the like, and the ASS light component generated due to Raman amplification of the own stage Raman amplifier. Then, when an input interruption of the signal light occurs, the light power monitored by the aforementioned light power monitor, becomes as shown in FIG. 13B, the light power corresponding to the ASS light component. Consequently, in order to perform a positive shutdown control for such an optical amplification apparatus unit, it becomes a subject to perform the correction in accordance with the aforementioned ASS light component, for the threshold value being the reference for judging an input interruption in the shutdown control in the conventional EDFA.
Furthermore, with the optical amplification apparatus which uses Raman amplification, since extremely high level excitation light is emitted into the optical fiber which constitutes the transmission path, there is the possibility that due to an open circuit of the transmission path or a disconnection of the connector, the excitation light may be emitted to the outside. In such a case, it is desirable to take measures such as, immediately lowering the excitation light power to a safe level, or switching off the drive condition of the excitation light source. However, to optical amplification apparatuses using Raman amplification, which have been proposed up to this date, the abovementioned measures have not been specifically applied.
The present invention addresses the above mentioned points, with the object of providing an optical amplification apparatus which uses Raman amplification and which can reliably judge an input interruption of signal light, and providing an optical amplification apparatus which can shut down the supply of excitation light in accordance with a judged input interruption of the signal light.
Therefore, an optical amplification apparatus of the present invention provided with first optical amplifying means for Raman amplifying signal light propagated through a Raman amplification medium by supplying excitation light to the Raman amplification medium, comprises input interruption detection means for detecting a noise light component due to the first optical amplifying means, and judging an input interruption of the signal light using the detection result. With such a construction, an input interruption detection of the signal light is performed taking into consideration an influence of amplified spontaneous Raman scattering light.
Furthermore, the construction may be such that the abovementioned optical amplification apparatus may comprise shutdown control means for shutting down supply of the excitation light when an input interruption of the signal light is judged by the input interruption detection means. With such a construction, when the input of signal light to the optical amplification apparatus is interrupted, supply of Raman excitation light is automatically shut down by the shut down control means, thus such a situation where high level excitation light is emitted to the outside can be avoided.
Furthermore, the construction may be such that the abovementioned optical amplification apparatus may comprise a second optical amplifying means for amplifying the signal light output from the first optical amplifying means. As a result, also in an optical amplification apparatus having a construction where the first and second optical amplifying means are combined, the reliable input interruption detection and shutdown control can be performed.
As a specific construction for the aforementioned optical amplification apparatus, the input interruption detection means includes an excitation light power detection section for detecting the excitation light power supplied to the Raman amplification medium, an input light power detection section for detecting the input light power to the second optical amplifying means, and a computation section for computing the noise light power due to the first optical amplifying means in accordance with the detection result of the excitation light power detection section, and performing correction of a relative level of a threshold value as a judgment reference for an input interruption and the input light power detected by the input light power detection section, in accordance with the computed noise light power, and judging an input interruption of the signal light when the input light power to the second optical amplifying means is less than the threshold value, and the shut down control means shuts down the supply of excitation light at least to the Raman amplification medium when an input interruption of the signal light is judged by the input interruption detection means. Furthermore, the shut down control means may also stop an optical amplifying operation of the second optical amplifying means when an input interruption of the signal light is judged by the input interruption detection means.
With such a construction, the power of the Raman excitation light is detected by the excitation light power detection section, and the input light power of the second optical amplifying means is detected by the input light power detection section, and each of the detection results are sent to the computation section. In the computation section, the noise light power due to Raman amplification is computed in accordance with the Raman excitation light power detected by the excitation light power detection section, and the correction processing of the threshold value as the judgment reference for an input interruption is performed, or correction (offset processing) of the input light power detected by the input light power detection section is performed, in accordance with the computation result. Then, when the input light power to the second optical amplifying means is less than the threshold value, an input interruption of the signal light is judged, and the shutdown control is performed by the shut down control means, for shutting off the supply of excitation light to the Raman amplification medium, and stopping the optical amplifying operation of the second optical amplifying means.
Furthermore, as another aspect of the optical amplification apparatus of the present invention, in an optical amplification apparatus provided with a first optical amplifying means for Raman amplifying signal light propagated through a Raman amplification medium which is connected thereto via a connector, by supplying excitation light to the Raman amplification medium, the first optical amplifying means comprises: a transmission excitation light power detection section for detecting the power of excitation light supplied to the Raman amplification medium, a reflection excitation light power detection section for detecting the power of reflection light generated as a result that the excitation light supplied to the Raman amplification medium is reflected by an end face of the connector, and a safety light control section for judging, based on each detection result from the transmission excitation light power detection section and the reflection excitation light power detection section, if the connector is normally connected, and when the connector is normally connected, setting the excitation light power to a predetermined level to enable Raman amplification, and when the connector is not normally connected, reducing the excitation light power to a safe level.
With such a construction, each power of the transmission light and the reflection light in the excitation light supplied to the Raman amplification medium via the connector, is respectively detected by the transmission excitation light power detection section and the reflection excitation light power detection section, and based on the detection results, the connection condition of the connector is judged by the safety light control section, thereby performing a so-called laser-safe light control for the Raman excitation light.